An ever-increasing number of viruses are being identified as the source of human disease. The better known diseases caused by viruses include chicken pox, measles, mumps, influenza, hepatitis, poliomyelitis, rabies, and now, of course, acquired immunodeficiency syndrome (AIDS). Them are, however, many more virus-related diseases. Indeed, viral infections are estimated to be responsible for more than sixty percent of human sickness occurring in developing countries.
In contrast to the success achieved by the use of antibiotics in the treatment of bacterial infections, efforts to treat viral infections have been largely ineffective. When individuals become infected, modem medicine can do little but ease the symptoms. Viral epidemics have only been avoided by treatment of uninfected individuals with vaccines.
The effort to treat viral infection has been hampered largely by the unique structural and functional characteristics of viruses. A retrovirus is essentially nucleic acid surrounded by a lipid-protein envelope. A virus invades a host cell and uses the host cell's machinery to replicate itself. The latter characteristic makes it especially difficult to find drugs which kill the virus and leave the host unharmed.
With a more detailed understanding of viral function, more sophisticated and promising approaches to treatment have been suggested. One approach has come from the recognition that the viral envelope proteins may be involved in binding to the host cell and, ultimately, penetration of the virus into the host cell. This approach involves the use of competing proteins or parts of proteins to block the binding or "fusion" event. An example of this approach can be found in U.S. Pat. No. 4,880,779 which describes inhibitory peptides to block retroviral fusion. Another approach has come from the recognition that some viruses use unique polymerases to replicate nucleic acid. This approach involves the use of competing, nucleotide derivatives to bind to the polymerase and stop replication. An example of this approach can be found in U.S. Pat. No. 4,916,122 which describes the use of synthetic deoxyuridine derivatives to block retroviral nucleic acid replication.
Another approach has emerged from a more in-depth understanding of the viral organization. For example, a complete sequencing of the AIDS virus genome indicates that the human immunodeficiency virus (HIV) genome exhibits the same overall gag-pol-env organization as other retroviruses. See L. Ratner, et al., Nature 313:277 (1985). The viral genes are initially translated into large precursor molecules that are subsequently processed into smaller, functional proteins by an enzyme known as HIV protease. See W. G. Farmerie, et al., Science 236:305 (1987). Since the processing by the protease is essential to the virus, it was suggested that inhibition of the protease could block virus maturation. See C. Debouck, et al., Proc. Nat. Acad. Sci. 84:8903 (1987), and N. E. Kohl, et al., Proc. Nat. Acad. Sci. 85:4686 (1988).
The type of compound that might inhibit retroviral proteases was suggested by the finding that a conserved sequence, Asp-Thr-Gly, of retroviral proteases is conserved in the active sites of aspartic proteases. See H. Toh, et al., Nature 315:691 (1985), and M. D. Power, et al., Science 231:1567 (1986). Indeed, detailed modelling as well as X-ray analysis indicates that retroviral enzymes could be examples of ancestral dimeric aspartic proteases. See L. H. Pear and W. R. Taylor, Nature 329:351 (1987); M. A. Navia, et al., Nature 337:615 (1989); R. Lapatto, et al., Nature 342:299 (1989); and A. Wlodawer, et al., Science 245:616 (1989).
Pepstatin A is a classic inhibitor of aspartic proteases. When tested for inhibition of HIV protease, pepstatin A was found to partially inhibit protein processing. See S. Seelmeier, et al., Proc. Nat. Acad. Sci. 85:6612 (1988); and P. Darke, et at., J. Biol. Chem. 264:2307 (1989). On the other hand, typical serine protease inhibitors have been shown not to inhibit HIV protease. See T. D. Meek, et al., Proc. Nat. Acad. Sci. 86:1841 (1989). This was strong evidence that the HIV protease was an aspartic protease.
Given the nature of the HIV protease, the most immediate approach to inhibiting the enzyme was to identify and design peptide substrates and peptide analogues that would act as inhibitors of the enzyme. A number of such substrates have been described. See M. L. Moore, et al., Biochem. Biophys. Res. Comm. 159:420 (1989); G. B. Dreyer, et al., Proc. Nat. Acad. Sci. 86:9752 (1989); T. D. Meek, et al., Nature 343:90 (1990); and A. G, Tomasselli, et al., Biochemistry 29:264 (1990).
Non-hydrolyzable peptide analogue substrates have also been described. N. A. Roberts, et al., Science 248:358 (1990) examine a family of related peptide inhibitors having non-hydrolyzable inserts. Similarly, T. J. McQuade, et al., Science 247:454, report on a variety of peptide inhibitors containing a hydroxyethylene isostere as a nonhydrolyzable, synthetic replacement for amino acids.
Peptide inhibitors are less than fully satisfactory as pharmacologic agents. As reported by H. P. Schnebli and N. J. Braun, Proteinase Inhibitors (Elsevier Science Publishers, 1986), many such inhibitors are readily degraded in vivo, and some do not penetrate cell membranes. For these reasons, most known peptide inhibitors require unreasonably high concentrations.
A handful of non-peptide inhibitors of HIV protease have been studied. Cerulenin, an antifungal antibiotic, is an example of a non-peptide inhibitor of HIV protease. See R. Pal, et al., Proc. Nat. Acad. Sci. 85:9283 (1988); J. J. Blumenstein, et al., Biochem. Biophys. Res. Comm. 163:980 (1989); and K. Moelling, et al., FEBS Letters 261:373 (1990). Unfortunately, cerulenin exhibits pronounced cytotoxicity.